The proposed study will attempt to elucidate age-related changes in thyroid function by using a longitudinal study with cotton rats. Specifically, this study will simultaneously examine the time course of changes in thyroxine (T4) concentration, triiodothyronine (T3) concentration, thyroid stimulating hormone (TSH) concentration, T3 uptake, T4 turnover rate, and basal metabolic rates in cotton rats of age 20 days (post-weaning) and 6, 12, 18, 24, 27, and 30 months (approximate maximum lifespan). In addition, T3 turnover rates and liver and brown fat T4 deiodination rates will be measured for the same periods.